DC motors have certain characteristics rendering them particularly suitable for drive systems requiring repeated start-stop operation. Typically, such drive systems are used in hoists, and elevators, many kinds of industrial machinery, and also in a variety of different vehicles.
However, it is well known that the power consumption for any given electric motor, during acceleration, and especially during acceleration from zero, is much greater than the power consumption required for steady state operation. The operation of such electric motors under conditions requiring repeated start-stop functions therefore involves a somewhat excessive power consumption.
Numerous attempts have been made to reduce this excessive power consumption during start-up, or acceleration from zero. One approach has been to provide an electric motor having an armature with multiple windings which may be connected or disconnected at various speeds.
A more complex proposal is the "Ward-Leonard" system, using both a motor and a generator, but it was expensive and not suitable for general use.
Other systems employ two or more separate windings, but with only limited success. Another approach altogether has been to provide for a two-speed operation, for example in elevator and hoist motors, where a slow and fast operation are required, by providing what are in effect two separate electric motors, with armatures actually fastened to the same shaft, in tandem.
However, such systems are of somewhat limited utility, and only provide for a very limited improvement over the use of a single motor.
In addition, since the system essentially involves the use of a simple two-speed operation, it is not generally speaking capable of providing a smooth continuous acceleration over a wide speed range, and in fact experiences the well known fluctuations in power consumption.
It is clearly desirable from a viewpoint of economy, and flexibility to provide an electric drive system which is capable of providing smooth continuous acceleration from zero, up to a desired maximum, with a minimum of fluctuations in power consumption, and at the same time providing such advantages within a reasonable cost.